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Absorber Thickness
More is always better; right? No, and it's still at least partially no for trap construction. Yes, adding 1-inch thickness to a 1-inch thick panel will probably always provide more absorption when using OC-703 rigid fiberglass or similar materials. However, as the thickness and the density increases the absorption characteristics will provide diminishing returns and become cost ineffective.

Some Reverberation Tests
The WEB contained references to some simple, but very telling, set of experiments in a small, rectangular room (roughly 16x11x8) with two doors and no closets or windows. The following configurations were tested:
  • Empty room - as a reference
  • 12 corner traps using 3-in thick OC-701,703,705 and -FRK types
  • 6 corner traps using 6-in thick OC-701,703,705 and -FRK types
  • A review of these tests is available here: Rigid Fiberglass Density Tests

Guess what
My own interpretation of the "waterfall" plots shown in the above test results are listed below. It is clear to me that corner bass traps are quite effective but measuring that effectiveness is complex; even using software. Also, the results appear to show that 'more is not always better':
  • My interpretation of the "waterfall" plots of the above tests appeared to show:
    • Corner bass traps definitely help - use them!
    • For 701/705 the -FRK layer does smooth the 100-200 Hz response region
    • OC-705 provides better 100-200 Hz response "smoothing" than OC-703
    • Ethan's data below 200 Hz is the only data on the Web showing <125 Hz performance
    • The -FRK layer shortens the 40 Hz reverb time about 150mS for -20dBc 40 Hz residuals
    • OC-705 provides roughly an additional 2-3dB response attenuation over OC-703
Surprise #1
  • Comparing equal volumes of absorption (12 traps at 3-in thick vs 6 traps at 6-in thick) using the same-size panels, thicker was NOT better!
  • In fact, covering more surface area was significantly better than just thicker traps
Surprise #2
  • 6-in thick OC-705-FRK provides significantly less response "smoothing" than 3-in thickness
  • With 6-in thick OC-703/705, -FRK makes the 40 Hz reverb times LONGER
Surprise #3
  • Filling the corner with additional fiberglass requires about 3x more fiberglass than just a 4-in straddling trap
  • However, the 3x additional fiberglass (and 3x cost) only improves bass absorption by about 25% !
Caution #1
Those tests did NOT observe the following and therefore caution is warranted when extrapolating those results to practical designs.
  • Test only the effect of "straddle" width - without changing the depth
  • Absorber edge exposure - what is the effect of a frame? The panels being tested had no support frames
  • Panel area's effect on using an -FRK layer - if the panel is only 16x48 (narrower) is the effect of using -FRK linear with area or is resonance changed dramatically making the -FRK ineffective ?
An Ideal Density?
With a lower density absorber (say OC-701) some sound energy is converted to heat, but most is still transmitted though the panel. With the OC-705 at much higher density, much more sound is converted to heat -- but some of the sound will actually be reflected (because of the higher density). So, there is a compromise. No test data appears to answer this question, but Ethan's experiment with OC-705 (at 6 pcf) suggests higher density (above 6 pcf, such as OC-707) would not be an improvement with panels of 3-in thickness.

An Ideal Thickness?
Consider 1-inch thick OC-703 rigid fiberglass. Sound will be attenuated but still much will get through. If the thickness increased to 3-feet, probably most all of the sound would be absorbed (at least for some frequencies). However, it would be expensive and sound attenuation is a linear fraction based on thickness.

For example, assume 2-inches reduced the sound energy by 66%; which means 34% transmission. Does 4-inches mean Zero? No. 4-inches is double the thickness but the attenuation is a ratio. A double layer of 4-inches means 34% of 34% or 11.6% transmission (88.4% absorption). Following this pattern, 6-inches would be 34% of 34% of 34% or 3.9% transmission (96.1% absorption). Now this seems near perfect, right? Well, yes, that is about 28dB which is very good considering the threshold of hearing differences is about 3dB.

How much is enough?
So, when is enough, enough? Must we always seek 30dB attenuation? Well, how much floor space and cash do you have? Let's calculate to compare assuming 2-inch thick panels are used to build other thicknesses. Each "unit" of fiberglass is a bundle of 6 each 24x48 inch panels so assume each bundle builds 6 traps for 48 sq-ft of treated room area. [Note: The exception is the PINK R8 which is 2.5-in and PINK R11 which is 3.5-in thick.]

Absorption Coefficients
First understand that measuring absorption is extremely difficult, even for a lab, as acoustic spaces for frequencies down to 10 Hz which do not "color" the measurement are very expensive to construct. So, the following table values were extracted from the Web for use in RELATIVE comparison, not ABSOLUTE values. The following considerations should be kept in mind:
  • A coefficient of 0.0 means sound is not attenuated and 1.0 means total absorption (generally never).
  • These numbers are all RELATIVE numbers for measurements of "random-incidence" absorption in a reverberation chamber therefore some measurements can return values >1.0.
  • Differences of < 0.15 are not significant in terms of absolute value -- it is the RELATIVE differences between type which count.
  • Measurements were made with the panel against a wall; in at least some cases
  • The Auralex material is 2-inches thick, but the surface is cut with a pyramid shape
  • The RockWool is the 2-in thick RW6 at 3.8 pcf density
  • Density is listed in (pcf) Pounds per Cubit Foot

Table 1: Random-incidence coefficients of absorption
   Auralex  OC-Pink
 OC-701 OC-703 OC-703-FRK
 Density(pcf)  1.5 1.8
 125 Hz
 0.13  0.16 0.11
 0.22 0.17 0.63
 0.16 0.60
 250 Hz
 0.18  0.50  0.60  0.67 0.86 0.56
 0.71  0.50
 500 Hz
 0.57  0.74  0.96  0.98 1.14  0.95  1.02  0.63
 1 kHz
 0.96  0.74  0.94  1.021.07
 0.79  1.01  0.82
 2 kHz
 1.03  0.73  0.92  0.98 1.02  0.60  0.99  0.45
 4 kHz
 0.98  0.82  0.82  1.000.98
 0.35 0.99

Absorption Plot dB
Admittedly, deciphering the above chart data can be confusing. Below is the same data in graphic form using the following guidelines:
  • Absorption is plotted as signal attenuation in dB (logarithmic like your ears)
  • Atten (dB) is calculated as 20 x Log( 1 / ( 1 - Coeff ) ); where the Coeff is from the table above
  • Since calculations can produce irrational results, the max attenuation is capped at 30 dB for plotting
  • Remember, 3 dB is roughly the smallest sound level change your ears easily detect !
  • Note: Performance at 125 Hz is NOT a predictor of frequencies below that !

Read the Plot
  • OC-701/703/705 are al very similar above 125 Hz
  • At 500 Hz and above, OC-701/703/705 all have similar absorption -- perhaps 30dB for 2-inches
  • Adding the -FRK membrane is not a good choice for broadband wall traps - especially above 1 kHz
  • RockWool is similar to OC-703/705 except for the density, weight and it's floppy and settles over time
  • The Pink wall insulation works well -- but it's harder to work with and sheds glass fibers. However, for very large volumes, it's a cheaper.
  • Rule-of-thumb: If you have space, fluffy-pink can work well. If space is limited use OC-701/703/705
  • Auralex foam is relatively useless below 1 kHz -- could only consider its use for Mids and flutter
OC-703/705 vs -FRK
The plots do NOT provide enough information to decide about using -FRK membrane, except:
  • -FRK is NOT helpful for regular wall trap absorbers
  • OC-703-FRK and OC-705-FRK are likely different below 125 Hz
Practical conclusions for corner bass traps:
  • For bass traps, cover as much surface area as possible
  • Use 3-4 inch thick traps instead of 6-in thick and "straddle" the corners
  • Leave dead airspace behind the panel rather than adding more fiberglass thickness
  • Expose the edges (minimal frames) if possible to maximize surface area
  • Use at least OC-703-FRK but no more than 4-inches thickness
  • OC-705-FRK provides improved "smoothing" of the response below 200 Hz
  • Use the -FRK membrane on the room side
  • If the budget is tight, use 2-in of OC-705-FRK with 2-in of OC-703 behind
  • Leave dead airspace behind the panel rather than adding more fiberglass thickness